This invention relates generally to interferometer controlled gas bearing X-Y-.theta. stage assemblies commonly utilized to facilitate the movement of an object along precisely orthogonal X and Y axes of motion and rotationally along a .theta.-axis about a Z direction which is orthogonal to the X and Y axes. The invention is particularly concerned with three axis interferometer control of such X-Y-.theta. stage assemblies. One typical use for such gas bearing X-Y-.theta. stage assemblies is to step and position a semiconductive wafer along X, Y and .theta. coordinates under the imaging lens of a step-and-repeat camera. Thus, different regions of the semiconductive wafer can be sequentially aligned and exposed in accordance with a pattern on a reticle. A typical prior art gas bearing X-Y stage assembly is shown in U.S. Pat. No. 3,722,996 entitled Optical Pattern Generator or Repeating Projector or the Like and issued Mar. 27, 1973, to Wayne L. Fox. Disclosed in that patent is an X-Y stage which is located directly with respect to a top reference surface of a base by three stem supported gas bearings. It is guided along a reference edge surface of a generally L-shaped frame extending across the top reference surface of the base and orthogonal to a reference edge surface of the base by a pair of guideway bearings attached to the X-Y stage. The L-shaped frame is guided along the reference edge surface of the base by another pair of guideway bearings attached to the L-shaped frame. The orthogonality of the resulting X and Y axes of motion of the X-Y stage of that patent is determined by the accuracy of the orthogonal relationship of the reference edge surface of the L-shaped frame and the guideway bearings attached to the L-shaped frame used to guide the L-shaped frame along the reference edge surface of the base. Although no .theta.-axis rotational capability is disclosed in that patent, .theta.-axis stages are employed with such X-Y stage assemblies to construct an X-Y-.theta. stage assembly, which assembly typically comprises a .theta.-axis stage mounted on top of such an X-Y stage.
The gas bearing X-Y stage assembly of U.S. Pat. No. 3,722,996 has the disadvantage that the gas bearings used as guideway bearings for guiding the X-Y stage and the L-shaped frame along their corresponding reference surfaces have axial compliance. This axial compliance between the gas bearings and their corresponding reference surfaces allows the flying heights of the gas bearings to vary dynamically in response to varying loads impressed by positioning drives coupled to the X-Y stage. What would be desirable is an improved gas bearing X-Y-.theta. stage assembly wherein the X, Y and .theta. axes of motion are independently and directly controlled without any guideway bearings at all.
Another disadvantage of that gas bearing X-Y stage is its inefficient use of space. As illustrated in FIGS. 2 and 3 of U.S. Pat. No. 3,722,996, the size of the envelope encompassing the X-Y stage and its extent of travel along the X and Y axes of motion is only a fraction of the total lateral area of the gas bearing X-Y stage assembly. What would be desirable is an improved gas bearing X-Y-.theta. stage assembly wherein the size of the envelope encompassing the X-Y-.theta. stage and its extent of travel along the X, Y and .theta. axes of motion is substantially the same as the total lateral area of the improved gas bearing X-Y-.theta. stage assembly.
Still another disadvantage of the gas bearing X-Y stage assembly of U.S. Pat. No. 3,722,996 is that the mechanical orthogonality of its X and Y axes of motion is determined by the orthogonality of a reference surface (the reference edge surface of the L-shaped frame) and guideway bearings (the two guideway bearings attached to the L-shaped frame used for guiding the L-shaped member along the reference edge surface of the base) of an intermediate stage element (the L-shaped frame). What would be desirable is an improved gas bearing X-Y-.theta. stage assembly wherein the orthogonality of its X and Y axes of motion and its .theta.-axis orientation are controlled with reference to a three-dimensional interferometer measuring system, whose frame of reference is the base of the gas bearing X-Y-.theta. stage assembly.
The X-Y stage assembly of U.S. Pat. No. 3,722,996 utilizes plane mirror interferometer control of its X and Y axes of motion. Plane mirror interferometer control of two axes of motion is thoroughly discussed in Application Note 197-1 entitled Laser Interferometers for Position Feedback, available from the Hewlett-Packard Co. of Palo Alto, Calif. The two controlled axes of motion in such systems are parallel to either of two mirrors which are mounted on the X-Y stage. Thus, motion along the controlled X and Y axes of motion of that X-Y stage is not necessarily parallel to the edge reference surfaces mentioned hereinbefore and controlled motion along either axis generally requires active control and motion of both X and Y axis drive systems. The orthogonality of the controlled X and Y axes of motion and the straightness of either of them depends on the accuracies relating to fabrication and mounting of the two plane mirrors. Further, each mirror must be slightly longer than the corresponding length of travel of the X-Y stage. This length makes the mirrors hard to fabricate and requires the lateral area of the X-Y stage upon which they are mounted to be physically larger than its extent of travel along the X and Y axes of motion. This, in turn, enlarges the size of the envelope encompassing the X-Y stage and its extent of travel.
One improvement to the foregoing is described in U.S. Pat. No. 4,311,390 entitled Laser Interferometer Measuring Apparatus, issued Jan. 19, 1982 to Edward H. Phillips and incorporated herein by reference, wherein a pair of mirrors are symmetrically positioned with respect to the Y-axis of motion, and first and second control circuits move an X-Y stage along precisely orthogonal X and Y axes of motion. However, the geometries associated with that patent generally require the mirrors to be longer and the X-Y stage larger than standard orthogonally disposed mirrors would require.
It would be desirable to have an improved gas bearing X-Y-.theta. stage assembly wherein a three dimensional X-Y-.theta. interferometer measurement and control system utilizes three relatively small targets mounted on an X-Y-.theta. stage, comprising a single moving stage element, and a linear motor capable of controlled motions in the corresponding three directions, to position the X-Y-.theta. stage. Thus, a three dimensional interferometer controlled X-Y-.theta. stage of minimum lateral area could be used for positioning a semiconductive wafer under the projection lens of a step-and-repeat camera. Such an X-Y-.theta. stage would have superior overall accuracy because all three axes of motion would be interferometer controlled, including the .theta.-axis. Further, the complexity, bulk and cost of a three element stage assembly (comprising an X-stage, a Y-stage and a .theta.-stage) normally utilized in such equipment would be eliminated.
Accordingly, it is a principal object of this invention to provide an improved gas bearing X-Y-.theta. stage assembly wherein motions of an X-Y-.theta. stage along X, Y and .theta. axes are controlled by a three dimensional X-Y-.theta. interferometer measurement and control system.
Another object of this invention is to provide an improved gas bearing X-Y-.theta. stage assembly wherein the X-Y-.theta. stage comprises a single moving stage element.
Another object of this invention is to provide an improved gas bearing X-Y-.theta. stage assembly wherein the orthogonality of its X and Y axes of motion and its .theta.-axis orientation are controlled with reference to a three dimensional X-Y-.theta. interferometer measurement sub-system, whose frame of reference is the base of the gas bearing X-Y-.theta. stage assembly.
Another object of this invention is to provide an improved gas bearing X-Y-.theta. stage assembly wherein the X, Y and .theta. axes of motion are independently controlled without any guideway bearings.
Another object of this invention is to provide an improved gas bearing X-Y-.theta. stage assembly wherein the size of the envelope encompassing the X-Y-.theta. stage and its extent of travel along the X, Y and .theta. axes of motion is substantially the same as the total lateral area of the improved gas bearing X-Y-.theta. stage assembly.
Still another object of this invention is to provide an improved gas bearing X-Y-.theta. stage assembly wherein the three dimensional X-Y-.theta. interferometer measurement sub-system utilizes three relatively small targets mounted on the X-Y-.theta. stage to minimize the lateral area of the X-Y-.theta. stage.
These and other objects, which will become apparent from an inspection of the accompanying drawings and a reading of the associated description, are accomplished according to illustrated preferred embodiments of the present invention by providing improved gas bearing X-Y-.theta. stage assemblies wherein an X-Y-.theta. stage comprises a single moving stage element. The X-Y-.theta. stage is supported by three gas bearings, driven by an X-Y-.theta. linear motor, and controlled by a three dimensional X-Y-.theta. interferometer measurement and control system. The three dimensional X-Y-.theta. interferometer measurement and control system uses a three dimensional X-Y-.theta. interferometer measurement sub-system wherein the three dimensional X-Y-.theta. interferometer sub-system utilizes three relatively small targets mounted on the X-Y-.theta. stage.
In a first improved gas bearing X-Y-.theta. stage assembly, the frame of reference for the three dimensional X-Y-.theta. interferometer measurement sub-system comprises first and second interferometers mounted on a first vacuum stabilized gas bearing slide, and a third interferometer mounted on a second vacuum stabilized gas bearing slide. The first and second vacuum stabilized gas bearing slides move along orthogonal X and Y axes reference surfaces of a base of the first improved gas bearing X-Y-.theta. stage assembly synchronously with respect to the X and Y axes locations of the X-Y-.theta. stage. The three dimensional X-Y-.theta. interferometer measurement and control system moves the X-Y-.theta. stage along X and Y axes of motion, whose orthogonality is determined by the orthogonality of the X and Y axes reference surfaces of the base, and controls .theta. orientation.
In a second improved gas bearing X-Y-.theta. stage assembly, the frame of reference for the three dimensional X-Y-.theta. interferometer measurement sub-system comprises first and second interferometers mounted on a first gas bearing slide, and a third interferometer mounted on a second gas bearing slide. The first and second gas bearing slides move along reference surfaces symmetrically positioned with respect to the Y-axis of motion synchronously with respect to the location of the X-Y-.theta. stage. The three dimensional X-Y-.theta. interferometer measurement and control system moves the X-Y-.theta. stage along precisely orthogonal X and Y axes of motion and controls orientation.
In a third improved gas bearing X-Y-.theta. stage assembly, the frame of reference for the three dimensional X-Y-.theta. interferometer measurement sub-system comprises three gas bearing spindle mounted interferometers. The three gas bearing spindles are rotationally oriented in a synchronous manner with respect to the location of the X-Y-.theta. stage. The three dimensional X-Y-.theta. interferometer measurement and control system moves the X-Y-.theta. stage along computed orthogonal X and Y axes of motion and controls orientation.